In relation to a conventional assembled battery formed by coupling multiple secondary battery cells, one of known techniques balances (equalizes) the voltages of the respective cells when the cells are being used or being charged. Specifically, this technique balances the respective voltages of the cells to reduce the dispersion of voltages (differences of the voltages), aiming at maximizing the battery capacity of the entire assembled battery. Balancing cell voltages while the assembled battery is being charged makes it possible to increase, until the highest voltage of a cell among the multiple cells included in the assembled battery exceeds the upper limit voltage, amounts of electric power charging the remaining cells. Similarly, balancing cell voltages while the assembled battery is being used makes it possible to increase usable electric power of the entire assembled battery. In other words, this can suppress reduction in available battery capacity.
There are two typical schemes to balance the cell voltage. One scheme is to discharge cells having high voltages to reduce the voltages thereof, so that the voltages of all the cells are balanced. One of the examples of this scheme has a configuration in which a resistor and a switch that consume charged electric power are connected in parallel to each cell and balances the voltages of the respective cells by controlling connection and disconnection of the respective switches (e.g., Patent Literature 1: Japanese Laid-Open Patent Publication No. 2003-309931).
The other scheme is to transfer electric power of cells having high voltages to cells having low voltages. For example, each of cells is disconnectably connected to a coil which is magnetically coupled with the each other coils. Electric power is transferred from cells having high voltages to cells having low voltages using electromagnetic induction (e.g., Patent Literature 2: Japanese Laid-Open Patent Publication No. 2006-166615). In either scheme, one or more cells having high voltage among all the cells are detected. The voltage of the high-voltage cells is reduced in order to balance (equalize) the voltage of the entire assembled battery.
Conventional cell-balancing control (equalization control) on cell voltage is carried out on the basis of the cell having the lowest voltage at the time.
For example, the technique disclosed in Patent Literature 1 specifies a cell having the lowest voltage when the total voltage of the assembled battery is equal to or lower than the target voltage. And the technique controls the time of discharging each remaining cell on the basis of the difference between the voltage of the remaining cell and the lowest-voltage cell.
The technique of Patent Literature 2 monitors voltages of the respective cells at any time and controls a time of activating the voltage balancer circuit on the basis of the difference between the highest voltage and the lowest voltage among the monitored voltages.
However, individual secondary battery cells are different in charging-discharging characteristics, so that the characteristics of the cells in the assembled battery are not uniformly degraded. This means that a change in a charged amount or a change in a charging rate may vary a cell having the lowest voltage among the multiple cells. Accordingly, if the voltages of multiple cells are to be balanced while the assembled battery is being used, the cell having the lowest voltage when the cell-balancing being carried out may not always have the lowest-voltage cell after the assembled battery is charged. There is a possibility that the dispersion of voltages rather increases because the cell-balancing is performed on the basis of the lowest voltage at the time.
The assembled batteries are used in vehicles in different manners according to the kind of vehicle, and the difference may affect the dispersion of voltages. For example, an electric vehicle relatively frequently undergoes external charging and consequently balancing of cell voltages can be carried out each time the vehicle undergoes external charging, so that the dispersion of voltage is less increased. The cell-balancing uses electric power from an external power source and therefore can escape from power shortage even if the cell-balancing takes a long time. Advantageously, an optimum state of cell voltages can be relatively easily kept.
In contrast, a plug-in hybrid vehicle, which is capable of charging the assembled battery with electric power generated by the vehicle per se, less frequently undergoes external charging than an electric vehicle. For the above, cell-balancing performed only when external charging as performed in an electric vehicle does not ensure sufficient frequency of the cell-balancing and therefore has a difficulty in preferably suppressing the dispersion of voltages. One solution to ensure sufficient frequency of the cell-balancing is to carry out the cell-balancing also when the vehicle is not undergoing external charging, which forces the vehicle to generate electric power to be consumed during the cell-balancing. Consequently, a longer time that the cell-balancing takes consumes a larger amount of electric power to degrade the fuel efficiency and the electric efficiency. Unfortunately, conventional manners of balancing of cell voltages have a problem that the dispersion of voltages of the cells is not appropriately controlled in some kinds of vehicle and/or in some manners of using an assembled battery.